datetime.tzinfo is abstract (#8908)

stubs/python-dateutil/dateutil/tz/_common.pyi

@@ -1,3 +1,4 @@
+import abc
 from datetime import datetime, timedelta, tzinfo
 from typing import ClassVar
 
@@ -8,7 +9,7 @@ class _DatetimeWithFold(datetime):
     @property
     def fold(self): ...
 
-class _tzinfo(tzinfo):
+class _tzinfo(tzinfo, metaclass=abc.ABCMeta):
     def is_ambiguous(self, dt: datetime) -> bool: ...
     def fromutc(self, dt: datetime) -> datetime: ...